1. Field of the Invention
This invention relates to electronic motor braking systems, and more particularly, to an electronic braking system having means for sensing that motor speed has decreased to a predetermined value.
2. Description of the Prior Art
Electronic or dynamic braking for slowing down and stopping electrical AC motors is well known in the motor art. It offers the advantage of being smooth and is not subject to the severe problems of rubbing surfaces used in mechanical braking systems. Dynamic braking employs a principal of removing an alternating current from the motor and applying an amplitude and time controlled DC current to the stator windings of the AC motor. The DC current produces a static field across the stator winding which generates a counterforce in the rotor of the AC motor to rapidly decelerate the motor without any mechanical braking. Preferably this dynamic braking is achieved without any modification to the AC motor or parallel connected AC motor brake solenoid. A mechanical brake can be employed to hold the motor in a stopped position and may also be used to help bring the motor to a complete stop.
Some of the prior art dynamic braking circuits employ a timing circuit which is connected between two of the AC power lines which are connected to the AC motor. A electro-mechanical brake release coil (AC solenoid) is connected across the same AC lines which supply power to the AC motor and when energized, the brake solenoid holds a spring loaded mechanical brake mounted on the motor in its released position. As long as power is uninterruptedly connected to the aforesaid motor lines, the brake release coil disengages the mechanical brake from the AC motor. After the timer has completed its timing sequence, the release coil is deenergized and the mechanical brake is engaged by the brake spring.
This timing circuit works well in conjunction with the electronic braking circuit to bring the motor to a low speed quickly. Unfortunately, varying load conditions, which are encountered in many motor applications make it more difficult to estimate the time required to reduce motor speed to a value at which the mechanical brake can engage the motor without causing excessive mechanical brake wear. What is needed is a device for sensing the decreasing speed or RPM of the AC motor and for causing the mechanical brake to engage the motor when the motor speed has decreased to a predetermined value.